Nutrient-driven colonization and weathering of silicates

In the subsurface biogeochemical environment, mineralogy, microbial ecology, and groundwater chemistry form an intimately linked system. Microorganisms cycle both carbon and metals from the groundwater and often change the groundwater chemistry and alter mineral solubility. However, when the groundwater is limited with respect to essential nutrients, some microbial communities develop strategies that allow them to extract these nutrients directly from the aquifer minerals. Organic ligands that chelate metals such as Fe, Al, and Si may play a part in the extraction of nutrients and subsequent destruction of silicate matrices.; The colonization and weathering of silicate minerals was examined with in situ field and laboratory microcosms using a native microbial consortium as a function of silicate composition. Parallel batch abiotic dissolution experiments were run to quantify nutrient-release and weathering. Nutrient-doped silicate glasses were used as a mineral-proxy to investigate nutrient-driven microbial colonization and weathering. An organic ligand, 3,4 dihydroxybenzoic acid (3,4 DHBA), capable of chelating Fe, A1 and Si, was added to selected laboratory experiments to investigate its role in mineral/glass weathering and microbial metabolism.; Field experiments were performed at the petroleum-contaminated aquifer at the USGS Toxics site near Bemidji, MN and the Lost River peat bog in northern Minnesota. Results from field microcosms in the anaerobic groundwater, where nutrients such as phosphorus (P) and iron (Fe) are scarce, show that colonization is influenced by the availability of essential nutrients in minerals and glass and that native subsurface microorganisms preferentially colonize and weather silicates that contain one or more of these nutrients. The implication is that nutrient-bearing silicates are colonized and destroyed, while non-nutrient bearing silicates are uncolonized and unweathered.; In laboratory microcosms using a native consortium, an increase in total biomass correlated to the presence of Pin the silicate. However, the degradation rate of toluene, benzene and 3,4 DHBA, and the release of Si, increased only when Fe was present. Increased weathering and biodegradation rates are enhanced when P is also present, possibly by increasing the biomass of a specific microbial population. It appears that organisms are targeting nutrient-bearing silicate surfaces and dissolving them in a microenvironment of reactive metabolic byproducts, possibly organic ligands. Microbial silicate dissolution and nutrient release appears to be enhanced by the presence of organic ligands. However, this interaction depends only on the nutritional needs of the native microbial consortium and does not necessarily follow traditional weathering sequences based on laboratory dissolution rates.